This invention relates to bearings. In particular, this invention relates to a bearing system and a flexible bearing bracket therefor.
Small fractional horsepower xe2x80x9cshaded polexe2x80x9d type motors are used in many applications, for example to provide air circulation in refrigeration systems. As is well known, shaded pole electric induction motors have a rotor comprising a rotor body bearing a shaft in rotationally fixed relation to the body. The rotor body is rotationally disposed within an opening in a magnetic stator assembly typically formed from a stack of aligned annular stator laminations. Electric field windings surrounding a portion of the stator magnetize the stator laminations to provide the required magnetic motive force for driving the rotor. In an air circulation system an impeller is mounted on the rotor shaft to drive the air flow.
In a conventional shaded pole motor the rotor shaft extends through a housing comprising brackets extending over each end of the rotor opening and secured, usually bolted, to the stator. The housing restrains the rotor body against substantial axial and radial displacement relative to the stator, and supports bearings which maintain the axial and radial alignment of the rotor shaft. The bearings thus maintain stability and alignment of the rotor while allowing for substantially free rotation of the rotor shaft. One example of such a motor is described in U.S. Pat. No. 5,287,030 issued Feb. 15, 1994 to Nutter, which is incorporated herein by reference.
Such fractional horsepower motors are particularly suitable for applications in which the motor runs for extended intervals over a prolonged period, which may be many years. As such the motor must be extremely durable, highly resistant to failure and preferably requires little maintenance over its useful life. The components which tend to be most problematic in achieving these parameters are the bearings, which are subject to persistent frictional contact with the rotating shaft over the life of the motor.
To maintain proper alignment of the rotor shaft, shaded pole type motors typically utilize spherical diameter, oil impregnated powdered metal bearings or ball bearings held in place by die cast aluminum or zinc bearing brackets. These types of bearings require constant exposure to a lubricant, which substantially limits the life of the motor. This problem is particularly acute in high temperature environments in which the oil used to lubricate the bearings dissipates over time, eventually causing catastrophic failure of the bearing system.
It is also known to press fit plastic or non-metallic journal bearings tightly to the bearing brackets. However, this type of bearing system requires machining after the press fitting operation, which significantly increases the manufacturing cost of the motor. Moreover, although a press fit journal bearing will remain in place in the bearing bracket during assembly, due to the interference fit between the bearing and the housing, the performance of the motor at times may be less than optimum because the fixed position of the bearing does not allow for even slight deviations in rotor shaft aligrunent. If the motor is jarred or bumped during operation, severe vibration and squealing can result because the bearing does not dampen the resulting vibrations due to disturbance of the shaft/bearing rotation.
These problems are particularly acute in the case of metal bearings supported by metal brackets, and precision machining of these components is therefore critical. There are also bearing systems which use a plastic bracket to support a metal bearing tightly fitted to the bracket in an interference fit. The system described in U.S. Pat. No. 5,287,030 uses a plastic bearing press fitted to a plastic bracket. However, in all of these systems the bracket is rigid, so that precise bearing alignment and adequate lubrication of the bearing remain critical to the proper operation of the motor, and the inability of the bearing to self align during operation reduces the useful life of the motor and generally causes the motor to operate less efficiently over time.
It is also known to use a plastic bearing press fitted into a metal bracket, but as the bearing is mounted the bracket closes the bearing inside diameter by the extent of the interference fit, which then necessitates precision machining of the inside diameter to restore adequate clearance for the rotor shaft. Also, the press fit operation causes the bearing to lose alignment during installation.
The present invention provides a bearing system comprising a bearing supported in a flexible bearing bracket, for example for use in fractional horsepower shaded pole type electric motors. In the preferred embodiment the bearing is insert molded into the bracket to produce a hybrid bracket/bearing assembly, which facilitates both the manufacture of the bearing system and assembly of the bearings to the motor.
In the preferred embodiment the bearing bracket is insert molded around a bearing, which ensures that the bearing is properly supported and will not misalign during assembly to the motor. The bearing system of the invention is self aligning due to the flexibility of the bearing bracket, which thus compensates for deviations in the radial alignment of the rotor shaft to maintain the optimum efficiency of the motor and reduce wear on the bearing, extending the life of the bearing system. In addition, the preferred embodiment employs a resilient elastomeric bearing bracket while efficiently damping vibrations which may be induced in the system.
In the preferred embodiment both the bracket and bearing are composed of a non-metallic material. The bearing may be composed of a high performance plastic which does not require lubrication, to prolong the life of the motor. Other aspects of the invention may be implemented in a bearing system that utilizes metal bearings.
In the preferred embodiment of the invention a flanged or bushing type bearing is provided with an opening having a bearing surface complimentary to the rotor shaft. A bearing bracket is molded around most of the flange and the hub of the bearing, creating a bearing receptacle that holds the bearing securely, with extremely close tolerances between the receptacle and the bearing. Preferably a rotation lock, for example flats distributed about the hub of the bearing, restrains the bearing against rotation within the bracket.
Because the elastomeric bracket is flexible, the bearing settles into the proper alignment with the rotor shaft by deflection of the bracket itself. In effect, the bracket serves the functions of both supporting the bearing and compensating for bearing misalignment, so that the bearing is able to adjust to accommodate deviations in the axial pitch of the rotor shaft, increasing the life of the bearing system.
In one embodiment the bearing bracket is molded with ribbed securing posts and/or complimentary sockets into which the securing posts are secured. This aspect of the invention simplifies assembly of the bearing brackets to the motor.
The present invention thus provides a bearing system for use with a motor (10) having a rotor shaft (16) and a rotor (16) rotating within an opening through a stator (20), comprising a bearing bracket (32) comprising a receptacle (40) surrounding a bearing (50) and supporting the bearing (50) in fixed relation to the bracket (32), wherein the bracket (32) is adapted to be mounted on the motor (10) such that the opening (56) in the bearing (50) is disposed in the vicinity of an axis of the rotor shaft (16), and wherein the bracket is sufficiently flexible that the rotor shaft (16) can deflect the bracket (32) so that the bearing 50 moves into alignment with an axis of the rotating shaft (16) but the bracket (32) is sufficiently rigid that the rotor (12) is maintained in spaced relation from the stator (20) during operation of the motor (10).
The present invention further provides motor (10) having a rotor shaft (16) and a rotor (16) rotating within an opening through a stator (20), having a bearing system comprising one or more bearings (50) each comprising an opening (56) having at least one bearing surface (58), for maintaining a radial alignment of the rotor shaft (16), and one or more bearing brackets (32) each comprising a receptacle (40) surrounding the bearing (50) and supporting the bearing (50) in fixed relation, wherein the brackets (32) are adapted to be mounted on the motor (10) such that the openings (56) in the bearings (50) are disposed on opposite ends of the stator (20) in the vicinity of an axis of the rotor shaft (16), and wherein the brackets are sufficiently flexible that the rotor shaft (16) can deflect the brackets (32) so that the bearings 50 move into alignment with an axis of the rotating shaft (16) but the brackets (32) are sufficiently rigid that the rotor (12) is maintained in spaced relation from the stator (20) during operation of the motor (10).